1. Field of the Invention
The present invention relates to an nuclear magnetic resonance imaging apparatus for taking arbitrary tomographic images of an object to be examined by utilizing a nuclear magnetic resonance phenomena.
2. Description of the Background Art
As is well known, a nuclear magnetic resonance imaging apparatus obtains arbitrary tomographic images of an object to be examined by detecting nuclear magnetic resonance signals induced by applying high-frequency excitation magnetic field (referred hereafter as RF-pulse) on the object placed in a static magnetic field, under a presence of gradient magnetic fields, and then analyzing the detected nuclear magnetic resonance signals to derive information on distributions of densities of particular nuclei and longitudinal and transverse relaxation times, from which arbitrary tomographic images are constructed by methods of image reconstruction processing.
A typical conventional nuclear magnetic resonance imaging apparatus is shown in FIG. 1. In this nuclear magnetic resonance imaging apparatus, a patient 101 is placed on a bed 102 extending into a bore 104 of a main magnet 103 in which a static magnetic field is generated by the main magnet 103. This main magnet 103 can be any one of a superconductive magnet, a normal temperature electromagnet, and a permanent magnet. Except when the main magnet 103 is a permanent magnet, this main magnet is magnetized and de-magnetized by a main magnet power source 105 through a lead cable 106. When the main magnet 103 is a superconductive magnet, this lead cable 106 will be removed after the magnetization in order to minimize consumption of liquid helium coolant, which is possible because the superconductive magnet can operate in a permanent current mode once magnetized.
The bore 104 is further inside equipped with a transceiver coil 107 for producing Rf-pulses and receiving nuclear magnetic resonance signals, a shim coil 108 for fine-adjustment for magnetic fields inside the bore 104, and a gradient coil 109 for producing gradient magnetic fields. The transceiver coil 107 comprises a transmitter coil and a receiver coil which are connected to a nuclear magnetic resonance signal receiving unit 110 and an RF-pulse producing unit 111, respectively. The gradient coil 109 comprises an X-coil, Y-coil, and Z-coil which are connected to X-power source 112, y-power source 113, and z-power source 114, respectively. All of the nuclear magnetic resonance signal receiving unit 110, RF-pulse producing unit 111, X-power source 112, Y-power source 113, and Z-power source 114 are controlled by a central processing unit 115 which also carries out all the analysis of the detected nuclear magnetic resonance signals. This central processing unit 115 is also connected to a display and operation unit 116 at which the obtained tomographic images are displayed and from which it is operated.
In such a nuclear magnetic resonance imaging apparatus, a imaging takes place in an uniform magnetic field region 200. In order to obtain a tomographic image for a complete slice of the patient 101, it is necessary for this uniform magnetic field region 200 to be a sphere wtih a diameter of 40 to 50 cm and less than 50 ppm field fluctuation. To meet such requirements, the main magnet 103 becomes as large as 2.4 m in length, 2 m in width, 2.4 m in height, and 5 to 6 tons in weight, when the main magnet 103 is a superconductive magnet. Moreover, since the main magnet 103 alone can produce a sphere of 40 to 50 cm diameter with only about few hundreds ppm field fluctuation at best, the additional use of the shim coil 108 is indispensable in achieving the required less than 50 ppm field fluctuation.
As briefly mentioned above, the imaging is carried out by detecting with the transceiver coil 107. The nuclear magnetic resonance signals are induced from a portion of the patient 101 placed within the uniform magnetic field region 200 by applying RF-pulses from the transceiver coil 107 along with the gradient magnetic fields from the gradient coil 109, which is under the control by the central processing unit 115. The central processing unit 115 then analyze the detected nuclear magnetic resonance signals to derive information on distributions of densities of particular nuclei and longitudinal and transverse relaxation times, from which arbitrary tomographic images to be displayed at the display and operation unit 116 are contructed by methods of image reconstruction processing.
Such a conventional nuclear magnetic resonance imaging apparatus presents the following problems.
First, it is necessary, because of the configuration described above, to insert the patient 101 into a narrow bore 104, which in a practical medical circumstances causes an unnecessary mental disturbances in the patient 101.
Secondly, it is necessary, again because of the configuration described above, to lay the patient 101 flat on the bed 102. However, in a case of a patient suffering from spinal hernia caused by excessive pressure on spine and spinal chord in a standing posture for instance, it is actually preferred to have an image taken in a standing posture. However, imaging in such a posture other than lying flat is not possible in the conventional nuclear magnetic resonance imaging apparatus described above.